This invention is related to a method and apparatus for sensing the O.sub.2 content of an exhaust gas of an automobile engine. The invention is more particularly related to an improved resistance type oxygen sensor having a titania resistor and a zirconia resistor.
Internal combustion engines, particularly automotive internal combustion engines, have exhaust gases which contain carbon monoxide, nitrogen oxides, and non-oxidized hydrocarbons, i.e., unburned or only partially burned hydrocarbons. All these substances contribute to air pollution. In order to reduce these substances which cause air pollution to a minimum value, it is necessary to clean the exhaust gases from the internal combustion engines as much as possible by effectively removing the largest possible quantity of these substances from the exhaust gases. This means that carbon monoxide and unburned hydrocarbons should be oxidized as completely as possible into their next higher oxidation stage, namely carbon dioxide and water (for the hydrocarbons), and the nitrogen-oxide compound should be converted to elemental nitrogen and oxygen.
Conversion of the noxious components of exhaust gases to non-poisonous compounds like carbon dioxide, nitrogen and water can be obtained by subjecting the exhuast gases to after-burning, i.e., subjecting them to temperatures above about 600.degree. C. while exposing them to catalysts. In order to succeed in this method, however, the composition of the exhaust gases must be so controlled that practically complete conversion of the exhaust gases to the non-poisonous compounds is possible. This means that the relationship of air to fuel is close to the stoichiometric value. As a measure of the stoichiometric value, the air number lambda has been used. At a value of lambda equal to one, the relationship of air to fuel is stoichiometric. If no excess oxygen is present which exceeds the equilibrium of the various possible reactions, lambda is less than one. If, however, lambda is greater than one, excess oxygen is present in the mixture. At lambda equal to one, the gas changes from a reducing to an oxidizing state.
To obtain a value of the air number lambda at approximately one requires that a sensing element be provided which is exposed to the exhaust gases and which determines oxygen content; this sensing element is then connected to a control device which controls the fuel or air supply and provides the correct ratio of fuel and air mixture to the internal combustion engine so that the exhaust gases will have as low a value of noxious components as possible.
Sensing elements which operate on the principle of elemental oxygen concentration and utilizing ion conductive cell electrodes have been used. The principles on which a solid electrolyte sensor operates is explained in great detail in U.S. Pat. No. Re. 28,792, reissued Apr. 27, 1976 (previous U.S. Pat. No. 3,400,054). This patent illustrates a solid electrolyte oxygen sensor which, when one side is exposed to exhaust gases and on the other side exposed to ambient air, provides an electrical signal which is a function of elemental oxygen concentration; both sides of the solid electrolyte are covered at least in part with platinum to form electrodes. The electrolyte is generally stabilized zirconia. Another example of such a sensor may be found in U.S. Pat. No. 3,978,006 entitled "Methods for Producing Oxygen-Sensing Element, Particularly for use with Internal Combustion Engine Exhaust Emission Analysis", issued Aug. -, 1976.
Another type of oxygen sensor is one wherein the electrical resistance of the sensor changes with the amount of oxygen present in the gas. This type of sensor is generally referred to as a resistance type sensor and the principle of operation of such a sensor is explained in U.S. Pat. No. 3,558,280 entitled "Solid-State Oxygen Gage" issued Jan. 22, 1971. The use of a titania resistance type sensor in an engine exhaust control system is also explained in U.S. Pat. No. 3,915,135 entitled "Circuit for Converting a Temperature Dependent Input Signal to a Temperature Independent Output Signal" issued Oct. 28, 1975.
The resistance type (titania) oxygen sensor has certain disadvantages. For instance, the titania sensor must operate over a range from 300.degree. C. to 900.degree. C., but the electrical resistance of the sensor, over the entire range, does not change in a manner that permits a delineation between a lean air-fuel mixture and a rich air-fuel mixture. Specifically, for a lean air-fuel mixture over the range of 300.degree. C. and 900.degree. C., the dc resistance of a titania sensor drops from 3.times.10.sup.9 ohms down to about 2.times.10.sup.4 ohms, while the dc resistance for a rich air-fuel mixture, over the same range, varies from 3.times.10.sup.6 ohms down to about 40 ohms. Therefore, the resistance characteristics for a rich and a lean mixture for the sensor overlap. Accordingly, for any temperature excursion exceeding about 250.degree. C. it is impossible, with an uncompensated titania sensor, to determine whether the air/fuel ratio is rich or lean. Of course, this is undesirable, as it would not be possible to control the air-fuel mixture because the titania type sensor cannot distinguish between a rich air to fuel mixture and a lean air to fuel mixture at temperature excursions above 250.degree. C.
An example of a gas sensor of titania ceramic material which includes a circuit for converting a temperature dependent input signal to a temperature independent output signal and control the air to fuel ratio of an automobile engine is shown in previously mentioned U.S. Pat. No. 3,915,135.